1. Field of the Invention
The present invention relates to a spreading and lap-forming machine for converting a web to a lap by folding in pleats.
2. Description of the Prior Art
In a machine of this type as disclosed, for example, in patent document FR-B-2,553,102, a web delivered by a card is first conveyed by a first conveyor-belt or so-called front belt to a pinching zone in which it is maintained between the first belt and a second belt and conveyed to an output carriage. The reciprocating motion of the output carriage above a receiving apron causes deposition of the web which said carriage discharges onto the apron in alternate accordion pleats. Since the apron moves in a direction parallel to the axes of the guide rollers carried by the output carriage, the successive pleats are relatively displaced along this direction and the web thus forms on the apron a continuous lap, the lateral edges of which are defined by the pleats. The beginning of the pinching zone is defined as a position in motion by an input carriage which performs a reciprocating movement. This movement is intended to ensure that the speed of admission of the web into the spreading machine is made compatible with the speed of unwinding of the web by the output carriage in spite of the reciprocating movements of said carriage. Thus the web, which has a fragile structure, is not disturbed either by packing or by stretching or by friction.
In machines of this type, the developed length of the closed paths followed by the conveyor-belts must be maintained constant in spite of the reciprocating movements of the carriages.
In some designs, the reciprocating motion of the input carriage is compensated by the contrary reciprocating motion of the output carriage and conversely by means of a suitable arrangement of the guide rollers (reference can be made, for example, to patent document FR-B-2,553,102).
In other designs (see FR-B-2,234,395 or U.S. Pat. No. 1,886,919), provision is made for auxiliary carriages which also carry out reciprocating movements of translation in order to selectively lengthen or shorten the conveyor-belts outside the pinching zone and thus to compensate for variations in length of the pinching zone.
As a general rule, in the zone in which it is in contact with the web, the front belt makes a 180-degree turn about one or a number of guide rollers carried by the input carriage. The web undergoes a centrifugal acceleration when it carries out this 180-degree turn. In consequence, the web has a tendency to become detached from the front belt at the level of the input carriage. This tendency becomes even more marked when the input carriage, by reason of its reciprocating movement, has a speed of displacement in a direction opposite to that of the speed of admission of the web located upstream. These detachments produce air pockets between the web and the conveyor-belts in the curved regions of the conveying path. The web formed by a light assembly of fibers is then liable to become dispersed, which has an adverse effect on homogeneity of the formed lap. Moreover, detachments may cause clogging in the web-pinching zone, which would clearly be undesirable. This drawback is particularly perceptible in the structure disclosed in patent document U.S. Pat. No. 1,886,919, in which the 180-degree turn is carried out without any external support.
In some designs such as the arrangement shown in the drawings of patent document FR-B-2,553,102, the 180-degree turn performed by the front belt on the input carriage is divided into two successive 90-degree turns each carried out around a respective guide roller, thus forming a substantially vertical web-conveying section between these two rollers. The second conveyor-belt or so-called rear belt also has a substantially vertical section which is delimited by two superposed guide rollers mounted on the input carriage and is adjacent to the vertical section of the front belt. The pinching zone therefore has a vertical region defined by the adjacent vertical sections of the front and rear belts. At relatively high speeds, accurate and jerk-free guiding of the web in this vertical region becomes very difficult, especially on account of vibrations of the conveyor-belts in their respective vertical sections. This disadvantage can be attenuated by reducing the length of these vertical sections but this entails the need to reduce the radius of curvature of the guide rollers which delimit said sections. However, one usually avoids the need to subject the conveyor-belts to turns having a small radius of curvature since such turns have an adverse effect on their mechanical resistance.
One of the objects of the present invention is to solve the problems mentioned above by proposing a novel spreading and lap-forming machine in which stability of conveyance of the web is improved.